1. Field of the Invention
The present invention relates to a plasma-chemical vapor-phase epitaxy system, i.e., a chemical vapor deposition system (hereinafter abbreviated as CVD system) which is suitable for the manufacture of a thin film for use in various electronic devices such as amorphous silicon solar cells, thin film semiconductors, and photo-sensors, as well as for use as semiconductor protective films, etc.
2. Description of the Prior Art
One example of a parallel plate type of plasma CVD system in the prior art is illustrated in FIG. 10. In this system, substrates 1 are disposed as opposed to a plate type RF (Radio-Frequency) electrode 5 equipped within a vacuum container 7.
Reaction gas 2 is made to pass through the RF electrode 5, and when an RF voltage is applied from an RF power supply 3 to the RF electrode 5, plasma is produced. Owing to this plasma, a reaction occurs and a thin film is formed on the substrates 1 as a result of CVD. In this figure, reference numeral 4 designates a shield, numeral 6 designates a substrate holder, numeral 8 designates a heater for heating substrates, and numeral 9 designates a thermocouple for detecting a temperature.
Also, one example of an RF induction coupling type plasma CVD system in the prior art is shown in FIG. 11. In this system, a quartz discharge tube 13 is mounted above a vacuum container 7a coaxially with the container, and around the discharge tube 13 is wound a discharging coil 12. In addition, substrates 1a are disposed on a substrate holder 6a at the center of the vacuum container 7a. A heater 8 heats the substrate, and a thermocouple 9 detects a temperature.
Reaction gas 2 is introduced from above the quartz discharge tube 13, and when electric power is applied from an RF power supply 3 to the discharging coil 12, plasma is produced within the discharging tube 13. Therefore, a thin film is formed on the substrates 1a as a result of CVD.
One example of a photo-CVD system in the prior art is illustrated in FIG. 12. A substrate 1b is disposed nearly at the center of a vacuum container 7b, and a light source 14 is disposed above the substrate 1b with a quartz glass window 15 interposed therebetween.
Reaction gas introduced into the vacuum container 7b is made to react by the energy of light radiated from the light source 14, whereby a thin film is formed on the substrate 1b as a result of CVD.
A characteristic property of the above-mentioned RF parallel plate type plasma CVD system in the prior art is that formation of a film having a large area of about 70 cm square is possible, and this system belongs to the most orthodox type of systems. However, due to the fact that the electrode is placed within the vacuum container, this system is subject to problems such as the release of impurities from the electrode and the production of particles caused by a peeling of films. In the case of the RF induction-coupling type plasma CVD system, although contaminants are not produced because of the fact that the discharging coil is disposed outside of the vacuum container, it is difficult to design the system to be capable of effecting deposition over a large area. Hence, it is not suitable as a system for mass-production, and it is solely used for research purposes. Although the photo-CVD system is not provided with an electrode and hence it can produce a film of very high quality, it presents a problem in that because a film is deposited on the glass window, light cannot pass through the window and formation of a film becomes impossible.
The above-described problems in the heretofore known CVD systems can be summarized as follows:
(1) In the case where an electrode for performing RF discharge is present in a vacuum, impurities from the electrode and a peeling of films would adversely affect a film on a substrate. PA0 (2) A system, in which excitation energy is supplied from the outside of the vacuum chamber by a coil instead of an RF discharging electrode, is difficult to adapt to large-area film formation. PA0 (3) In a photo-CVD system, although a film of high quality can be formed, formation of the film becomes impossible when the glass plate reaches an opaque state.